1. Field of the Invention
The present invention relates to a fuel cell and to a fuel cell assembly.
2. Description of the Related Art
To obtain energy of the next generation, there have recently been proposed a variety of fuel cells by containing a stack of fuel cells in a container.
FIG. 3 is a diagram illustrating a stack of conventional solid oxide fuel cells. In this stack of cells, a plurality of fuel cell 1 are regularly arranged, a collector member 5 made of a metal felt is interposed between one fuel cell 1a and another neighboring fuel cell 1b, and a fuel-electrode 7 of one fuel-cell 1a is electrically connected to an oxygen-electrode (air-electrode) 11 of another fuel cell 1b. 
The fuel cell1 (1a, 1b) is constituted by a solid electrolyte 9 and the oxygen-electrode 11 composed of electrically conducting ceramics, that are provided in this order on the outer peripheral surface of the fuel-electrode of a cylindrical cermet (of which the interior serves as a fuel gas passage). An interconnect 13 is provided on the surface of the fuel-electrode 7 covered with neither the solid electrolyte 9 nor the oxygen-electrode 11. As will be obvious from FIG. 3, the interconnect 13 is electrically connected to the fuel-electrode 7 so will not to be connected to the oxygen-electrode 11.
The interconnect 13 is made of electrically conducting ceramics which is subject to be little degraded with an oxygen-containing gas such as a fuel gas or the air. The electrically conducting ceramics must be so dense as to reliably shut the fuel gas flowing inside the fuel-electrode 7 off the oxygen-containing gas flowing outside the oxygen-electrode 11.
The collector member 5 provided between the neighboring fuel cells 1a and 1b is electrically connected to the fuel-electrode 7 of the one fuel cell 1a through the interconnect 13, and is further directly connected to the oxygen-electrode 11 of the other fuel cell 1b, so that the neighboring fuel cells are connected together in series.
A fuel cell assembly is constituted by containing the cell stack having the above-mentioned structure in a container, and generates electricity at 600 to 1000° C. by flowing the fuel gas (hydrogen) into the fuel rod 7 and flowing the air (oxygen) into the oxygen-electrode 11.
In the fuel cell constituting the above-mentioned fuel cell assembly, in general, the fuel-electrode 7 is formed of Ni and yttria stabilized zirconia (YSZ), the solid electrolyte 9 is formed of ZrO2(YSZ) containing Y2O3, and the oxygen-electrode 11 is constituted by a perovskite composite oxide of the type of lanthanum manganate.
As a method of producing the above fuel cell, there has been known a so-called co-firing method which forms the fuel cell by co-firing the fuel-electrode 7 and the solid electrolyte 9. The co-firing method is a very simple process requiring a decreased number of steps for production, featuring increased yield in the production of cells, and is advantageous for decreasing the cost.
However, the conventional fuel cells are accompanied by such problems that the fuel-electrode 7 is cracked when the fuel-electrode 7 and the solid electrolyte 9 are co-fired, and the solid electrolyte 9 peels off the fuel-electrode 7 which is a support member. That is, the solid electrolyte 9 is formed of ZrO2 containing Y2O3 having a coefficient of thermal expansion of 10.8×10−6/° C. Here, however, the fuel-electrode 7 supporting the solid electrolyte 9 contains Ni having a coefficient of thermal expansion of 16.3×10−6/° C. which is very larger than that of YSZ. At the time of co-firing, therefore, there develops a large difference in the thermal expansion between the solid electrolyte 9 and the fuel-electrode 7 supporting it. As a result, there occur such problems as cracking of the fuel-electrode 7 and peeling of the solid electrolyte 9.
In order to solve the above problems, further, it has been proposed to use mulite (3Al2O3.2SiO2) and spinel (MgAl2O4, CaAl2O4) and to form the fuel-electrode 7 by using them in combination with Ni (see Japanese Unexamined Patent Publication (Kokai) No. 29574/1995).
According to the above proposal, the coefficient of thermal expansion of the fuel-electrode 7 is brought close to the coefficient of thermal expansion of the solid electrolyte 9, making it possible to suppress the occurrence of cracks in the fuel-electrode 7 and the peeling of the fuel-electrode 7 from the solid electrolyte 9 at the time of co-firing. There, however, arouses a new problem in that the components such as Mg, Al and Si in the fuel-electrode diffuse into the solid electrolyte 9 at the time of co-firing, whereby the ionic conductivity of the solid electrolyte 9 decreases and the generating performance of the fuel cell decreases.